Synthetic polyamide substrates, such as carpeting, upholstery fabric and the like, are subject to staining by a variety of agents, e.g., foods and beverages. An especially troublesome staining agent is FD&C Red Dye No. 40, commonly found in soft drink preparations. Different types of treatments have been proposed to deal with staining problems. One approach is to apply a highly fluorinated polymer to the substrate. Another is to use a composition containing a sulfonated phenol-formaldehyde condensation product.
For example, Blyth and Ucci, in U.S. Pat. No. 4,592,940, describe the preparation of stain-resistant nylon carpet by immersing the carpet in an aqueous solution of a sulfonated condensation polymer wherein at least 40% of the polymer units contain --SO.sub.3 X radicals and at least 40% of the polymer units contain sulfone linkages.
On the other hand, in U.S. Pat. No. 4,501,591, Ucci and Blyth disclose continuously dyeing polyamide carpet fibers in the presence of an alkali metal meta silicate and a sulfonated phenol-formaldehyde condensation product so as to impart stain resistance to the dyed carpet. They report that in experiments in which either the alkali meta silicate or condensation product was omitted from the dyeing process, or in which silicates other than the alkali metal meta silicates were used, they failed to obtain stain-resistant carpets (Column 8, lines 4-12).
Frickenhaus et al., in U.S. Pat. No. 3,790,344, disclose a process for improving fastness to wet processing of dyeings of synthetic polyamide textile materials with anionic or cationic dye stuffs. After dyeing the textile materials, Frickenhaus et al. treated the dyed materials with condensation products prepared from 4,4'-dioxydiphenylsulphon, formaldehyde and either a phenol sulfonic acid, a naphthalene sulfonic acid, sodium sulfite, or sodium hydrogen sulfite.
However, sulfonated phenol-formaldehyde condensation products are themselves subject to discoloration; commonly they turn yellow. Yellowing problems are described by W. H. Hemmpel in a Mar. 19, 1982 article in America's Textiles, entitled Reversible Yellowing Not Finisher's Fault. Hemmpel attributes yellowing to exposure of a phenol-based finish to nitrogen oxides and/or ultraviolet radiation. Critchley et al., Heat Resistant Polymers; Technologically Useful Materials, Plenum Press, N. Y. 1983, state that the thermo-oxidative stability of phenol-formaldehyde resins can be improved by etherifying or esterifying the phenolic hydroxyl group. Orito et al., in Japanese Published Patent Application Topkukai 48-1214, describe preparing flame-retardant filaments by (A) reacting (i) a phenol-containing compound, (ii) benzoquanamine, melamine or a methylol derivative thereof and (iii) formaldehyde; (B) forming filaments by melt-spinning the resulting polymer and (C) reacting the filaments with an esterifying or etherifying agent so as to effect color change in the filaments. In an example, soaking the filaments in acetic anhydride for five days caused their color to change from pink to pale yellow.
Meister et al., in U.K. patent specification 1 291 784, disclose condensation products of 4,4'-dihydroxydiphenylsulphone, diarylether sulphonic acids, and formaldehyde, and the use of such condensation products as tanning agents and as agents for improving the fastness to wet processing of dyeings obtained on synthetic polyamides within anionic and/or cationic dyestuffs. Meister et al. disclose that by preparing their condensation products in an acid pH range, leathers tanned with the condensation products showed practically no yellowing after 100 hours exposure to light in Xenotest apparatus.